Old one
but it used to make me giggle:
At Southampton Uni, there was a notice on a door saying 'This door is alarmed' under which some wag had written 'Well calm it down then!'
(
splorp ®,
Tue 4 Nov 2003, 13:42,
archived)
much of the same
On the toilet door in the Queens pub in sunny Bradford was the sign "caution wet floor" to which some wag had written underneath "this is not an instruction"
(
fr13day has everything required to be a Penguin,
Tue 4 Nov 2003, 16:27,
archived)